Our soils remain our single most valuable asset. This is not just because of the potential market value of land – a soil’s ability to produce governs your current and future productivity.
Soils can easily account for a 30% difference in crop yield and how we manage those soils can either increase or decrease that difference.
All productive soils depend on the optimum integration of their physical, chemical and biological properties.
Far too many soils have been subjected to chemical fertilisation only and some see soil physical characteristics as being influenced by subsoiling.
Productive land needs well-structured soil which has adequate porosity for roots to grow and breathe, adequate organic matter to fuel mineralisation and feed the biology that restructures the soil and then adequate nutrient availability from both soil and bag to feed the higher yield potential.
AHDB report No 576
Most people accept the importance of organic matter in the soil and its potential benefits.
However, understanding or being able to explain what exactly it does in the soil, or how it benefits the soil, is considerably more difficult.
A recent report published by the AHDB in the UK entitled Improvement of soil structure and crop yield by adding organic matter to soil looked at the consequences of adding different sources of organic matter or amendments to the soil.
The objective was to measure and explain the effects that the different products had on the soil.
The AHDB is the Agriculture and Horticulture Development Board and some may know it as the former HGCA (Home Grown Cereals Authority).
It is charged with collecting levies from growers and others within the industry and apportioning some of these resources to focused research, marketing, etc.
This recent report featured a series of in-depth studies across different types of organic matter amendments.
Having more organic matter in the soil can have many effects, depending a bit on what is added
It then measured very many aspects of the potential impact of these amendments on the soil.
The on-going challenge is that the results and benefits found were quite variable and variable yield benefits were not readily explained.
Having more organic matter in the soil can have many effects, depending a bit on what is added. One hopes for increased earthworm activity to drive improvements in soil structure as a direct consequence of their activity.
This would then lead to increased activity in the armies of associated organisms that work with them in the soil and so increased microorganism numbers and activity is another objective.
In altering soil structure, one would hope for increased porosity in the soil. This would lead to easier root growth and improved water percolation through the soil.
Mineralisation level should be increased with higher organic matter and this should lead to improved nutrient availability and higher yield potential.
All of these aspects do not necessarily interact in any specific order but they do form part of the tapestry on which improved productivity arises from healthier soils.
Research results
Yield and harvest results
In general, yield was increased where organic amendments were used on fields and it took less nitrogen to hit optimum yield.
The level of yield increase encountered varied with the crop grown but it was generally of the magnitude of 10%.
However, it took at least two consecutive years of application before the yield increases became statistically significant. But the benefits did not stop there.
These and other trials found that the benefits appear to continue, albeit at a reduced level, for between two and perhaps up to five years after product applications cease.
The researchers concluded that carbon content, or perhaps energy or cellulose content, is a better guide to the magnitude of these benefits than just total mass or even the dry matter of the organic sources added
In an effort to understand what the different organic matter sources were doing in the soil, the research looked at what the different organic matter sources were delivering.
This was examined in terms of their delivery of carbon or cellulose in terms of energy.
The researchers concluded that carbon content, or perhaps energy or cellulose content, is a better guide to the magnitude of these benefits than just total mass or even the dry matter of the organic sources added.
Some older experiments appeared to show just over two tonnes of carbon per hectare per year to be an optimum rate of amendment but newer trials put this at 6t C/ha/yr, while much older trials (1960s and 1970s) found 1t C/ha/yr.
Is this a consequence of the falling soil OM levels or the increased productivity and architecture of modern varieties?
Recent trials
Recent trials showed that grain nitrogen content was largely unaffected by added organic matter sources but the majority of these sources were not high-N manures like pig or poultry manures.
However, thousand grain weight was increased slightly, possibly as a result of more power in the ground later in the season.
This would also mask possible higher grain-N content, as more starch in grains would dilute the N content.
It is also worth noting that oil concentration in oilseed rape was increased slightly through the use of these OM additions.
While this increase was frequently not significant, the benefit was of the order of 3% which would be of benefit to a crusher if captured.
It is noteworthy that the benefits of adding OM tended to be higher on lighter or sandy soils than on heavier soils that contain more clay.
However, it was also noted that the various benefits persist longer in clay soils.
This could be related to the greater ease with which organic matter might be degraded from lighter soils.
This research also looked specifically at adding OM sources to land which had been using different establishment techniques.
Yield increases were found in the reduced tillage site but these tended to be lower than with plough-based establishment.
The report suggested that the benefits of adding the OM sources used in these experiments were somewhat similar in magnitude to reducing tillage intensity.
The authors wondered if perhaps both practices show their benefits, at least partly, by means of improved soil structure in the surface of the soil.
Earthworms
Earthworms are generally seen as being at the top of the pile of the various organisms that do good in the soil.
We are told that earthworm numbers are significantly encouraged by the addition of organic matter, regardless of its form, to the soil.
However, this report did not fully support this theory. In the two separate longer-term experiments, there was no evidence of any consistent increase in earthworm numbers following the use of organic amendments.
Neither was there evidence of any rate or formulation of amendment impacting on population size, total biomass or biodiversity of earthworms.
There were some effects found but these were generally not consistent across site or year.
Basically this study did not find evidence of any inherent stimulation of worm populations by organic amendments and yet yield effects occurred.
It was noted that the min-tilled site had higher numbers and total biomass of earthworms than the conventionally tilled field.
It was also stated in the report that the effect of earthworm numbers on plant productivity tended to be variable and effects were found at a range of frequencies.
However, it also stated that effects were more consistent when frequencies was above 400 earthworms per square metre.
This threshold was exceeded at times in some plots, but not consistently.
These findings led the researchers to conclude there may be some form of physical bottleneck in soil systems, associated with soil structural features, which acted to prevent earthworm numbers from responding to the addition of the amendments.
One might also wonder if some other factor is at play which prevented numbers from responding, such as being very low to begin with or even the efficacy of the systems used to assess earthworm numbers.
Microbiology
The researchers measured a range of soil microbiology impacts in many of these experiments but found that there was little solid correlation between amendment application and microbial biomass. However, rotation did have a significant effect on microbial biomass.
General comments
Our past reliance on chemical fertilisers has meant that farmers forgot about the role that fresh organic materials play in feeding soil organisms and the whole biological system.
These amendments are much more useful for soil organisms than the remaining soil organic matter because it is higher in energy.
And this is why their use is associated with increased soil organism activity which, in turn, should lead to improved soil structure.
Other elements of this research suggested that the response to applied N changes where FYM had been applied and that potential yield was found to increase where it was used.
This has long been my contention for wanting to feed soils with any form of organic material and especially amendments that provide nutrition for both the soil organisms and crop plants.
The report authors argue that the success of plant breeders in producing varieties with higher harvest index (more grain and less straw in the total biomass produced) should support the more efficient use of nutrients.
This implies that the additional nutrient that might be supplied from amendments should not be the reason for higher yield delivery where amendments had been used.
That said, it seems likely to me that additional nutrient available from the soil may help support the development of higher yield potential at a time when the crop is not being fed by chemical fertilisers. Or might any yield benefit be a result of unknown compounds being produced in the soil during processes like mineralisation which could act as growth promoting substances to drive plant growth?
At one trial site, the resistance to soil penetration was significantly reduced following the addition of amendment.
The authors ask if this is a consequence of an improvement in soil structure with an associated reduction in soil density? However, the precise mechanisms as to how these benefits affect plant growth remain unclear.
The authors concluded that amendments can improve yield, soil structure and the mass of soil organisms.
However, they were not able to associate these factors with one another, except from the fact that they all come about as a result of amending the soil.
Experimentation is continuing that might help to answer many remaining questions about the mechanisms of yield increases.
Summary research points
Some experiments also showed that it is more difficult to get macro-organisms to thrive on higher clay soils than on light sandy/silty soils and that this may be a factor in limiting short term benefits.
However, it seemed that the benefits to yield potential tend to persist longer in clay soils than in lighter land.
US midwest crop tour did not lower yield expectations